The goal of the project is to determine what percentage of heart muscle cells is capable of DNA synthesis and division at different developmental stages and thus to be able to estimate what amount of functional regeneration and repair can be expected after congenital or surgical damage to the heart during development and growth. Myocardial cells from chick embryos and post-hatching chicks of different developmental ages are grown as monolayer cell cultures or tissue cultures. Proliferating cells are labeled as they enter the DNA synthesis portion of the cell cycle by 3H-thymidine, and either embedded and sectioned for electron microscopy or treated with fluorescein-conjugated antibody against the muscle protein myosin. Both preparations are coated with photographic emulsion for autoradiography, and cells are scored for 3H-thymidine-labeled muclei and fluorescent myofibrils by fluorescence microscopy or thick (myosin) filaments by electron microscopy. From this data the proliferative fraction of heart muscle cells at each developmental age can be calculated. By plotting proliferative fraction versus age, we can determine at what point in development heart muscle cells become a non-dividing population. Similar experiments on chick hearts labeled in vivo with 3H-thymidine injections will determine the extent to which myocardial cell proliferation in vivo differs from that in vitro. BIBLIOGRAPHIC REFERENCES: Goode, D. Mitosis of embryonic heart muscle cells in vitro. An immunofluorescence and ultrastructural study. Cytobiologie 11: 203-229 (1975). Salmon, E.D., D. Goode, T. K. Maugel, and D.B. Bonar. Pressure-induced depolymerization of spindle microtubules. III. Differential stability in HeLa cells. J. Cell Biol. 60: 443-454 (1976).